J. Phys. Chem. lQ83, 87, 1524-1529
1524
and barium sulfate." Although these "one-shot" nucleations resemble a single pulse of the Morgan reaction, temporal oscillations do not occur because the small particles of condensed phase do not clear from the solution as rapidly as bubbles do. Implications for Gravitational Effects. If the phenomenon under consideration is indeed strongly influenced by the big difference in density between gas and liquid, it is amusing to speculate about a Morgan reaction system in an orbiting satellite. The analysis as developed here suggests that there should be no more than one pulse of pressure increase. Oscillations should also be inhibited in a centrifuge just as they are by vigorous stirring. On the other hand, a chemical system that generated a liquid or solid phase by initially homogeneous reaction might exhibit repetitive pulsed nucleations if it were centrifuged sufficiently strongly. Acknowledgment. This research was supported in part by a Grant from the National Science Foundation to the University of Oregon.
Symbols Useds A
c
surface area of a bubble, cm2 [ CO(soln)] = concentration of homogeneously (16) Causey, R. L.; Mazo, R. M.Anal. Chem. 1961,34, 163C-3. (17) Nielsen, A. E. Acta Chem. Scand. 1961, 15, 441-2.
KP,= equilibrium concentration at P, largest radius of bubble considered to be in cell j subscript from 1 to M defining the designation of a cell rate of nucleation, cm-3 s-' Boltzmann constant, erg K-' rate constant for consumption of formic acid, s-l rate constant for bubble escape to gas, s-' rate constant for transport between bubble and solution, cm s-' number of cells number of moles of gas in a bubble number of bubbles in cell j pressure inside a bubble of radius r , dyn cm-2 or atm hydrostatic pressure on solution bubble growth coefficient for cell j , s-' radius of bubble, cm gas constant, erg mol-' K-* time, s temperature, K volume of bubble, cm3 volume of solution, cm3 work necessary to create a bubble, erg 6+, + 6-, = width of cell j , cm Henry's law constant, mol cm-3 atm-' surface tension, dyn cm-' k,[HCOOH](soln)] = rate of consumption of formic acid, mol s-l Registry No. Formic acid, 64-18-6carbon monoxide, 630-08-0.
Enhanced Catalytic Activity of Cobalt Tetraphenylporphyrin on Titanium Dioxide by Evacuatlon at Elevated Temperatures for Intensifying the Complex-Support Interaction Isao Mochlda,
Katsuya Suetsugu, Hlroshl Fujltsu, and Kenjlro Takeshlta
Research Institute of Industrial Science, Kyushu Unlversw 86, Kasuga 816, Japan (Receivd: June 14, 1982; I n Final Form: November 12, 1982)
Remarkable catalytic activity of cobalt tetraphenylporphyrin (CoTPP) supported on TiOz was found to be developed by the evacuation of the catalyst at 200 "C for the reduction and decomposition reactions of nitric oxide which were observed with a circulating reactor. The reduction reaction was detectable to as low as 50 "C, and the conversion of nitric oxide into nitrogen was completed at 100 "C within 4 and 1 h with hydrogen and carbon monoxide, respectively. The decomposition of nitric oxide into nitrous oxide without any reductant was also observable at 100 "C. Such an activity increase of roughly 20 times due to the evacuation was related to the increased capacity for the adsorption of hydrogen and the further activation of carbon monoxide and nitric oxide. These enhancements can be ascribed to the pronounced electron transfer from the support to the complex (to produce active Co2-' and anion radical which can be active sites for carbon monoxide and nitric oxide, and hydrogen, respectively) strengthened by the partial dehydration of the support which may produce on its surface the coordinatively unsaturated titanium ion surrounded by an appropriate number of TiOH groups. A spillover mechanism is proposed in the decomposition of nitric oxide where oxygen produced probably on the central metal ion of the complex can be transferred to the support. Introduction The roles of supports in the efficient performance of heterogeneous catalyses are multifold. The support disperses and stabilizes the catalytic species over its surface to provide a large effective surface area during the catalytic reaction. In addition to these rather physical consequences, a strong chemical or electronic interaction between the support and the catalytic species can modify the As intrinsic catalytic activity or selectivity of the (1) Boundart, M. Adu. Catal. 1969, 20, 153.
far as the metal oxide support is concerned, the modification of the catalytic species through contact with the support should be governed according to the boundarylayer4 and electronic6theories of semiconductors. Stong metal-support interactions have been reported in the methanation, adsorption of carbon monoxide and hydrogen (2) Cinneide, A. D. 0.; Clarke, J. K. A. Catal. Reu. 1972, 7, 213. (3) van Hardeveld, R.; Hartog, F. Adu. Catal. 1972, 22, 75. (4) Solymosi, F. CataE. Rev. 1967, I , 233. (5) Slinkin, A. A.; Fedorovskaya, E. A. Russ. Chem. Reu. (Engl. Transl.) 1971, 40, 860.
0022-3654/83/2087-1524$01.50/00 1983 American Chemical Society
Enhanced Catalytic Activity of CoTPP/TiO,
over palladium or platinum supported on oxide ~ u p p o r t s . ~ The extent of interaction has been ascribed to the positions of the Fermi level of the support, although no direct evidence is p r e ~ e n t . 4 Recently, ~ ~ ~ ~ organometallic complexes such as metal-carbonyl clusters, which could maintain their structure on the support, were used in some catalytic reactions in their supported form.1° We have reported" the significantly enhanced activity of CoTPP supported on T i 0 2 for the reduction of nitric oxide with hydrogen and carbon monoxide. Such an enhancement is another example of the strong support-catalyst interaction. In the present study, the catalytic activity of CoTPP/ Ti02 evacuated at 200 "C was investigated for the reduction of nitric oxide with carbon monoxide as well as hydrogen, assuming that the treatment removing an appropriate number of the hydroxyl groups on T i 0 2 may be favorable for the complexaupport interaction through an electron transfer by producing coordinatively unsaturated sites on the support. Adsorption of substrates and ESR and ESCA spectroscopies of the evacuated CoTPP/Ti02 were also investigated to quantify the interaction from viewpoints other than kinetic ones.
Experimental Section Catalyst. CoTPP was synthesized according to Adler's method.12J3 The complex was examined by elemental analysis and UV spectroscopy (Shimazu UV-202). Titanium dioxide (Ti02),which was prepared from titanium oxysulfate and calcined a t 300 "C (BET surface area: 156 m2/g), was offered by Titan Industry, Ube, Japan. T i 0 2 was added to a red-purple solution of CoTPP in benzene and stirred at room temperature for several hours. By removing the solvent under reduced pressure, we prepared a green slurry of CoTPP/Ti02 (5 wt 90CoTPP). Metal-free tetraphenylporphyrin supported on T i 0 2 (H2TPP/Ti02)was prepared in the same manner. Procedure. We measured the catalytic activity for the reduction of nitric oxide with hydrogen and carbon monoxide a t 50 and 100 "C, using a conventional circulating reactor (800 mL) with a fixed catalyst bed in which a thermowell was located. The circulation rate was ca. 500 mL/min at atmospheric pressure, which was attained with a magnetically driven piston. The evacuation or heat treatment in some atmospheres was performed a t several temperatures before the reaction. Partial pressures of NO, H2, CO, and Ar (internal standard) were 2,60,60, and 1 cmHg, respectively. Yields of N2, N20, and NH3 were determined by periodic sampling (5 mL) and analyzed with a gas chromatograph, using columns packed with molecular sieve 13X,Porapak-Q, and 1-hendecanol/liquid paraffin on Flusin T, respectively. The amount of adsorbed NO was estimated from the difference of mass balance as for nitrogen in the closed reactor system. The adsorption of hydrogen was observed volumetrically with a glass apparatus equipped with a mercury manom(6)Vannice, M. A. J. Catal. 1975, 40, 129. 1978, (7)Tauster, S. J.; Fung, S. C.; Garten, R. L. J.Am. Chem. SOC. -inn. - -, 17n. - . -. (8)Deu Otter, G. L.; Dautzenberg, F. M. J. Catal. 1978, 53, 116. (9)Schwab, G.M. Adu. Catal. 1978,27, 1. (10)Smith, A. K.; Theolier, A.; Basset, J. M.; Vgo, R. J . Am. Chem. SOC. 1978,100, 2590. (11)Mochida, I.; Tsuji, K.; Suetsugu, K.; Fujitau, H.; Takeshita, K. J . Phys. Chem. 1980,84, 3159;J. Chem. Soc., Chem. Commun. 1982,166. Mochida. I.: Suetsueu. K.: Fuiitsu. H.: Takeshita.. K.:. Tsuii. K.: " I .Sanara. . Y.; Ohyoshi, A. J. &tal. 1982, 77; 519. (12)Adler, A. D.;Longo, F. R.; Finarellii, J. D.; Goldmacher, J.; h o u r , J.; Korsakoff, L.J. Org. Chem. 1967, 32, 476. (13)Adler, A. D.; Longo, F. R.; Kampas, F.; Kim, J. J. Znorg. Nucl. Chem. 1970, 32,2443.
The Journal of Physlcal Chemistry, Vol. 87, No. 9, 1983
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Reaction time (hr) Figure 1. Reduction profiles of NO with H, over CoTPP/TiO, at 100 "C: (A) the catalyst was evacuated at 100 "C before the reaction; (e)the catalyst was evacuated at 200 "C before the reaction. Initiil pressures: P,, = 2 cmHg, P H 2= 60 cmHg.
eter and connected to a vacuum line and a gas reservoir. After the adsorbent (1g) was evacuated a t the fixed temperature for 1 h, hydrogen (50 cmHg) was introduced so that it was in contact with the adsorbent, its uptake being monitored by means of a mercury manometer. Spectroscopy. ESR spectra were obtained with a JEOL JES-FE1X spectrometer a t 77 K. The g values were determined by direct comparison with a Mn2+ line in a standard MgO sample. The relative radical concentration in the CoTPP/Ti02 or TiOz was estimated from the relative height of the ESR signal to that of a Mn2+line in a standard MgO sample. The same pretreatment for the catalyst applied before the reaction was performed in an ESR sample tube before the measurement. ESCA spectra were obtained with a Shimazu ESCA 750 (Mg Ka,10 kV, 30 mA) at room temperature. The binding energies were corrected as to the C1, line of the contaminated carbon as often performed.
Results Catalytic Activity of CoTPP/Ti02after Evacuation for the Reduction of Nitric Oxide or Nitrous Oxide with Hydrogen. Reduction profiles of nitric oxide a t 100 "C with hydrogen over CoTPP/Ti02 catalyst evacuated at 100 and 200 "C (abbreviated as CoTPP/Ti02-100 and CoTPP/Ti02-200, respectively) are compared in Figure 1. The adsorption of nitric oxide over CoTPP/Ti02-100 leveled off within 10 min after the contact of the gas with the catalyst, where the level of adsorption was 22% of the introduced amount as shown in Figure 1A. The level stayed constant for 5 h until nitric oxide in the gas phase decreased to 60% of the introduced amount, producing 18% nitrous oxide. The adsorption over CoTPP/Ti02-200 very rapidly reached the level of 20% within 3 min, being followed by a sharp decrease in accordance with the decrease of the oxide in the gas phase as shown in Figure 1B. Within 1.5 h, nitric oxide was completely converted into nitrous oxide. Molecular nitrogen appeared in the gas phase only after the complete conversion of nitric oxide. Within 4 h, nitrous oxide derived from nitric oxide was completely converted into molecular nitrogen. Thus, the catalytic activity of CoTPP/Ti02 was remarkably enhanced by the evacuation at 200 "C. The rate of formation of nitrous oxide over CoTPP/Ti02-200 (2.2 X low4mol/ (g of catalyst-h) was 20 times larger than that over mol/(g of cata1yst.h). AlCoTPP/Ti02-100 (1.2 X though the rates were quite different, similar reduction profiles on both catalysts indicate a strictly successive
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The Journal of Physical Chemlstty, Vol. 87, No. 9, 1983
Mochida et ai.
n -
0
1
Reaction time
2
3 4 5 0 1 2 Reaction time ( h r )
3
4
5
Flgure 2. Reductlon profiles of N,O with H, over CoTPP/TiO, at 150 "C: (A) the catalyst was evacuated at 150 "C before the reaction; (B) the catalyst was evacuated at 200 "C before the reaction. Initial pressures: PNZO= 1 cmHg, P H 2= 60 cmHg.
Figure 3. Decomposltion profiles of NO over CoTPP/TiO, at 100 "C: (A) the catalyst was evacuated at 100 "C before the reaction; (6)the catalyst was evacuated at 200 "C before the reaction. Initial pressure of NO is 2 cmHg.
transformation of NO N20 N2 as reported previously.11 The catalytic activities of CoTPP/Ti02 for the reduction of nitrous oxide with hydrogen are shown in Figure 2, where the reaction temperature was 150 "C. The favorable effect of evacuation at 200 "C on the catalytic activity was also obvious in this case. CoTPP/Ti02-200 required less than 0.5 h for the complete conversion of nitrous oxide, whereas CoTPP/Ti02-100 required 3 h. The catalytic activity of CoTPP/Ti02-200 in a series of runs is observed a t 100 "C, where the same catalyst was repeatedly used after the evacuation a t 100 "C. The activity definitely decreased in the second run. Further decrease was observed for the third run after the same treatment. However, the evacuation of the same catalyst a t 200 "C after repeated use restored the activity to that of the virgin one. The treatment of the catalyst in dry oxygen a t room temperature after the evacuation at 200 "C was indifferent to the activity; however, oxygen treatment a t 200 "C burned CoTPP on Ti02, producing a large amount of carbon dioxide. The burned catalyst showed no activity for the reduction and adsorption of nitric oxide a t all, ruling out the possibility that the metal oxide on Ti02, even if produced through the decomposition of the complex, is a catalytically active species. The catalyst evacuated at 100 "C lost water of 20 mg/(g of catalyst) by further evacuation a t 200 "C. When the same amount of water was adsorbed on CoTPP/Ti02-200, the rate of N20 formation a t 100 OC was lowered to 8.2 X lo4 mol/(g of catalyst-h), which is slightly smaller than that over CoTPP/Ti02-100, indicating that water removed during the evacuation at 200 "C can be intimately related to the activity enhancement. Decomposition of Nitric Oxide in the Absence of Hydrogen. Decomposition profiles of nitric oxide catalyzed by CoTPP/Ti02 a t 100 "C are shown in Figure 3. The initial rate of formation of nitrous oxide from nitric oxide over CoTPP/Ti02-200 (Figure 3B) was 10 times that over CoTPP/Ti02-100. The high activity of CoTPP/Ti02-200 for the decomposition was quite remarkable since unsupported CoTPP showed essentially no activity at all a t this temperature. Slow but distinct production of molecular nitrogen, while nitric oxide was still present, should be noted, in contrast to the reaction in the presence of hydrogen. However, nitrous oxide was the only product a t the very initial stage of the reaction when the catalytic
TABLE I: Amounts of Adsorbed NO or N,O over CoTPP/TiO, during the Reaction"
- -
mole ratio of adsorbed NO/CoTPPb evacuationC a t reaction temp
reactant NO NO NO N,O
reaction temp, without "C H, 50 100 150 150
1.77 0.92 0.76d 0.06
evacuationC a t 200 "C
with
without
H,
H,
H,
1.84
1.64
with
0.82
l.Old
1.77 e
0.28d 0.04
e
e
0.16
0.12
Catalyst: 4 g; 2 cmHg of NO and/or 6 0 cmHg of H,; 1 cmHg of N,O and/or 60 cmHg of H,. Adsorption a t 2 h after the reaction started. Evacuation for 3 h. Significant reduction of NO took place before the measurement. e Adsorbed NO disappeared within 1 h. a
turnover number (mole ratio of reacted NO/CoTPP) was below 1. Molecular nitrogen is suggested to be produced successively from nitrous oxide. The competitive transformation of nitrous oxide may not strictly be forbidden in the reaction without hydrogen. The fate of the oxygen produced in this reaction should be studied in relation to the catalytic turnover number (N20produced per CoTPP). Neither molecular oxygen, nitrogen dioxide, carbon dioxide, carbon monoxide, nor water was detected in the gas phase, and the total turnover number can be 12, indicating that the residual oxygen should be retained on the catalyst system. No change in the oxidation state of the central metal ion was observed after the reaction by ESR or UV. The supporting T i 0 2 can act as an oxygen sink, since reduced or photoirradiated TiOz was known to contain a certain amount of Ti3+ ion which can adsorb the oxygen in 02-or 0-form.14-17 Oxygen presumably produced on CoTPP may be transferred to the active site of TiOz via the spillover mechanism.18 The evacuation a t 200 "C restored the activity for the decomposition, whereas 100 OC was not high enough. There may be a threshold tem(14) Cornu, P. F.; van Hoff, J. H. C.; Pluijm, F. J.; Schuit, G. C. A. Discuss. Faraday SOC.1966, 41, 290. (15) Naccache, C.; Meriaudeau, P.; Che, M.; Tench, A. J. Trans. Faradav SOC. 1971.67.506. (16) Munuera, G.; Rives-Amau, V.; Saucedo, A. J. Chem. SOC.,Faraday Trans. I 1978, 75, 736, 748. (17) Cunningham, J.; Morrissey, D. J.; Goold, E. L. J. Catal. 1978,53, f,X (18) Sermon, P. A.; Bond, G. C. Catal. Reu. 1974, 8, 211.
Enhanced Catalytic Activity of CoTPP/TiO,
TABLE 11: Amounts of Adsorbed H, at Room Temperature after Evacuation at Several Temperaturesu adsorbed H, evacuation ,umol/(g adsorbent temp, "C of catalyst) CoTPP/TiO, rtb 0.4 CoTPP/TiO, 100 6.1 CoTPP/TiO, 200 19.6 H,TPP/TiO, rtb 0.0 H,TPP/TiO, 100 9.0 H,TPP/TiO, 200 13.5 TiO, rtb 0.0 TiO, 200 0.4 a PH,: 50 cmHg; 1 g of adsorbent was evacuated for 1h at evacuation temperature. The amount of adsorption was measured 2 h after the introduction of hydrogen. rt = room temperature.
The
Journal of Physical Chemistry, Vol. 87, No. 9,
0 1 2 3 4 5
0
1983
1527
1
Reaction time (hr)
perature for the desorption of oxygen. Adsorption of Nitric Oxide and Nitrous Oxide on CoTPPITiO,. Table I summarizes the amounts of adsorbed nitric oxide and nitrous oxide on the catalyst with and without hydrogen. At 50 "C, more than one molecule of nitric oxide was adsorbed on one molecule of CoTPP supported on TiO, regardless of the evacuation temperature before a significant amount of nitric oxide reacted. Since the Ti02and H2TPP/Ti02 adsorbed nitric oxide to an extent much inferior to that of CoTPP/Ti02, although the electron transfer from T i 0 2 to H2TPP took place to allow significant hydrogen adsorption, the major nitric oxide can be assumed to be adsorbed on CoTPP supported on Ti02, suggesting that some of the nitric oxide coordinates in the twin or dimer f ~ r m . 'At ~~ 100 ~ and ~ 150 "C, less than 1 mol of nitric oxide was adsorbed on 1 mol of CoTPP. It is noted that the amount of nitric oxide adsorbed at these temperatures was increased by raising the evacuation temperature. This trend is more significant for the adsorption of nitrous oxide on CoTPP/Ti02. The amount of adsorbed nitric oxide decreased slightly in the presence of hydrogen at 100 "C and quite definitely at 150 "C even if the decrease of partial pressure of nitric oxide due to the reaction was taken into account as described in Table I, indicating that the central metal ion, the adsorption site for nitric oxide, may suffer some electronic modification through the T system of the ligand where hydrogen is adsorbed as described later. The evacuation a t 200 "C accelerated the adsorption of nitric oxide slightly but favorably for the catalysis. The acceleration was significant with nitrous oxide. A modification of the adsorption site favorable for the activation of nitrogen oxides is strongly suggested. Adsorption of Hydrogen on CoTPP/Ti02. Adsorption of hydrogen on CoTPP/Ti02, H2TPP/Ti02, and Ti02at room temperature is summarized in Table 11. Although Ti02and CoTPP showed essentially no adsorption activity for hydrogen even after evacuation a t 200 "C, CoTPP/ TiOz adsorbed a significant amount of hydrogen after the evacuation, increasing along with higher evacuation temperature. Its remarkable increase was observable with the evacuation a t 200 OC. The evacuation at 200 "C allowed adsorption of 19.6 pmol of hydrogen/(g of catalyst). A comparable amount of hydrogen was adsorbed on H2TPP/TiOz after the evacuation, indicating that the porphyrin ring electronically modified by T i 0 2 provides an adsorption site for hydrogen as reported before" and (19) Pierpont, C. G.; Van Deveer, D. G.; Durland, W.; Eisenberg, R.
J. Am. Chem. SOC.1970,92,4760.
(20) Wayland, B. B.; Miiewicz, J. V. J. Chem. SOC.,Chem. Commun. 1976, 1015.
Flgure 4. Reduction profiles of NO with CO over CoTPP/TiO, at 100 "C: (A) the catalyst was evacuated at 100 "C before the reaction; (B) the catalyst was evacuated at 200 "C before the reaction. Initial pressures: P,, = 2 cmHg, P,, = 60 cmHg.
that the evacuation at 200 "C enhanced markedly its ability to adsorb hydrogen. Catalytic Activity for the Reduction of Nitric Oxide with Carbon Monoxide. The catalytic activities of CoTPP/Ti02-100and CoTPP/Ti02-200 for the reduction of nitric oxide with carbon monoxide a t 100 "C are illustrated in Figure 4, where the partial pressures of nitric oxide and carbon monoxide were 2 and 60 cmHg, respectively. It took 40 min to complete the conversion of nitric oxide to nitrous oxide over CoTPP/Ti02-100, whereas only 20 min over CoTPP/Ti02-200. The latter catalyst completely converted the nitrous oxide thus produced to nitrogen within l h. In contrast, the former catalyst converted only 50% after 5 h. Thus, the evacuation at 200 "C is quite effective for the activation of carbon monoxide. Even at 50 "C the evacuated catalyst converted nitric oxide into nitrous oxide within several days. The rates of nitrous oxide and nitrogen formation were 1.8 x and 9.0 x lo4 mol/(g of catalyst-h), respectively, on CoTPP/TiO2-200. These values are 8 times those observed with hydrogen over the same catalyst. It is of value to note that CoTPP/Ti02-200 maintained its high activity in repeated runs a t 100 "C. The reaction orders in nitric oxide and carbon monoxide were zero and 0.9, respectively, on CoTPP/Ti02-200. Although 1mol of carbon monoxide was adsorbed on 1mol of the complex of CoTPP/Ti02-200, it was hardly adsorbed where nitric oxide was present during the reaction. Although the adsorption of nitric oxide during the reaction was not accurately measured because of its too rapid disappearance, its amount was estimated to reach almost 0.8 mol/(mol of complex) at the very initial stage of the reaction. It appeared slightly less than that in the presence of hydrogen, shown in Table I. Spectroscopic Characterization of CoTPP/ TiOz. The ESR spectrum of CoTPP/Ti02 after the evacuation at 200 OC was observed a t 77 K under vacuum. CoTPP/Ti02, as well as H2TPP/Ti02,has been reported in a previous paper'l to exhibit an ESR signal (g = 2.0024, line width = 3.5 G ) which was assigned to the anion radical of the complex, its unpaired electron locating in the porphyrin a orbital. The relative concentrations of radical in CoTPP/TiO2 catalyst evacuated at some temperatures or treated in some atmospheres after the evacuation at 200 "C are summerized in Table 111. Raising the evacuation temperature increased markedly the radical concentration. The radical concentration decreased with the introduction
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The Journal of Physical Chemistry, Vol. 87, No. 9, 1983
TABLE 111: Relative Concentration of Anion Radicals on CoTPP/TiO,a
Mochida et al. (Reduction)
pretreatment conditions temp, "C
atmosphere
rtb 100 200 rtb 100 rtb
evacuation evacuation evacuation H, (60 cmHg) H2(60cmHg) NO ( 2 cmHg)
time, h 1 1 1 2 1 2
re1 concn 1 2.44 5.96
1 0.86 0.29
1
I/d"o.
H20
0.90
a ESR was observed a t 77 K. Catalyst amount: 80 mg. g value of the peak (corrected t o Mn2+): 2.0023 * 0.0001. b r t = room temperature.
of hydrogen, particularly a t 100 "C, where the concentration was reduced to one-seventh. Similar phenomena were observed for H2TPP/Ti02. The dissociative adsorption of hydrogen on the anion radical of the catalyst may be responsible. The dissociative adsorption of hydrogen was also suggested by the reaction order of 0.6 in hydrogen. The radical concentration was also decreased slightly by the introduction of nitric oxide (pressure: 2 cmHg) which is believed to be adsorbed on the central metal ion. The adsorbed nitric oxide may interact indirectly with the porphyrin ring through the central metal ion. Ti02 showed ESR signals by itself after the evacuation at room temperature, although their intensities were quite small compared with those of CoTPP/Ti02 The signals were assigned to the free electron (g = 2.002) and the Ti3+ ion (gL = 1.946, gl, = 1.882). Both intensities increased significantly after the evacuation at 200 "C. Although their concentrations were still much smaller than that of anion radicals produced after the evacuation, high-temperature evacuation may activate Ti02which may, in turn, enhance the activity of CoTPP. ESCA did not give spectra of high quality because of low intensity; nevertheless the binding energies (electronvolta) of cobalt ion in the complex were read as follows: CoTPP CoTPPiTiO
C0,~312 781.0 780.2
Cozpl
2
796.4 795.6
N,, 397.3 398.6
These values showed clearly the shifts of binding energies caused by the support, indicating the lower oxidation state of the Co ion in CoTPP/Ti02. The binding energies of nitrogen in the complex were also shifted by the support to the higher energy direction, indicating lower electron density. Although unfortunately carbon atoms of the ligand could not be distinguished in ESCA because of the large conjugated system, the anion radical may locate on the carbon atoms of the pyrrole rings and methene bridges in the ligand."
Discussion In a previous paper,l' we reported that supporting CoTPP on Ti02 enhanced significantly its catalytic activity for the reduction of nitric oxide with hydrogen. The enhancement appeared to be related to the electron transfer from the support to the complex which produced anion radicals in the porphyrin ring as revealed by ESR measurement. The radicals could be sites for hydrogen activation. The chemical shift of ESCA in the present study indicates that the transfer also provides partial reduction of the central metal ion as designated by C O ~ -which ~ , is favorable for the activation of adsorbed nitric oxide in its (21) Sundbom, M. Acta Chem. Scand. 1968,22, 1317.
' Ti Oi"' ( Decomposition )
0= porphyrin ring anion radicals Flgure 5. Reaction scheme for the reduction and decomposition of NO over CoTPP/TiO, evacuated at 200 "C before the reaction.
bent form through the back-donation to its T* orbital because of enriched d electrons on the metal ion. Such back-donation may weaken the N-O bond. The evacuation of the catalyst a t 200 "C in the present study further enhanced the activation of nitric oxide, carbon monoxide, and hydrogen, allowing considerable extent of reaction to as low as 50 "C. The evacuation was also revealed to increase the extent of hydrogen adsorption. Nitric oxide was found to be decomposed into molecular nitrogen without hydrogen by the catalyst after the evacuation at 200 "C, although no oxygen was detected in the gas phase, indicating so much activation of nitric oxide, or loosing of ita N-0 bond. The free electrons identified as being located on the porphyrin ring increased significantly in concentration after the evacuation, suggesting enhanced electron transfer from the support to the complex by the evacuation treatment. Consequently, the increases of free electrons and C Ocan ~ be~ responsible for the enhancement of the catalytic and adsorption activities of CoTPP. The heat treatment of CoTPP/Ti02-100 at 200 "C under vacuum enhances catalytic activity and removes water. Addition of water decreased the activity back to that before the treatment. Contact with dry gases, including oxygen at room temperature, exhibited no effect. Reduction with carbon monoxide where no water was produced by any means kept the high reactivity in repeated runs. Such experimental facts strongly suggest that partial dehydration of T i 0 2 is the major cause of activity enhancement. According to the detailed IR study of Munuera et al.16 on the hydrated state of Ti02, the oxide heated in the atmosphere at 300 "C contained much adsorbed water and many hydroxyl groups on its surface; however, the evacuation above 200 "C removed these species to a major extent without changing the surface area. Such a dehydration reaction may produce coordinatively unsaturated titanium ions which can be active sites for the electron donation to CoTPP. Recently, Enriquetz and Fraisoard22 reported that the electron-donor sites against tetracyanoethylene were produced on Ti02 by partial dehydration, reaching a maximum number with evacuation at ca. 200 "C. Surface titanium ion surrounded by an appropriate number of TiOH groups can be the donor site, as Peri23proposed that such a situation on alumina pro(22) Enriquetz, M. A.; Fraisoard, J. P. J . Catal. 1982, 74, 77.
J. Phys. Chem. 1983, 87, 1529-1534
1529
duced active sites, although the latter sites are acidic. The evacuation changed the color of TiOz from white to gray, indicating the formation of F centers according to the defect of lattice oxygen." However, oxygen treatment at room temperature hardly influenced the activity, ruling this out as a major contribution. The reaction scheme of the reduction with hydrogen and decomposition of nitric oxide on CoTPP/Ti02 evacuated at 200 OC can be illustrated as shown in Figure 5. The reduction scheme essentially shows the activations of both nitric oxide and hydrogen which are performed on the central metal ion and the porphyrin ring, respectively. The reaction kinetics suggest the dissociative adsorption of hydrogen%and Rideal-type participation of a second nitric oxide, although one nitric oxide was firmly adsorbed on the catalyst. The decomposition scheme illustrates the oxidation of the complex by nitric oxide. However, the spillover of the oxygen atom left on the central metal ion onto the supporting titanium dioxide which eventually provides the
adsorption site for oxygen may allow the extra extent of the reaction beyond the stoichiometry of the cobalt ions. When the support is saturated by oxygen, the decomposition reaction may stop. It is another interesting feature of the complex-upport interaction. Removing the oxygen left a t 200 "C can regenerate the decomposition activity of CoTPP/Ti02. The reaction scheme of the reaction with carbon monoxide appears different from that with hydrogen in terms of their activation. The reaction orders and adsorption amounts suggest that nitric oxide and carbon monoxide are adsorbed commonly on the central metal ion of the complex, but the former much more strongly. The very great reactivity of carbon monoxide in comparison with hydrogen indicates that the adsorbed species is very much activated to attract the oxygen atom from nitric oxide which has itself been adsorbed in the activated bent form. The partially reduced cobalt ion on Ti02 may enrich the valence electrons of the carbon atom of the carbon monoxide through the back-donation to its K* orbital.
(23) Peri, J. B. J . Phys. Chem. 1965, 69, 211, 220. (24) Meyer, W. 2. Tech. Phys. 1935, 16, 355. (25) Mochida, I.; Fujitau, H.; Takeshita, K.; Tsuji, K. R o c . Int. Congr. Catal., 7th 1980, F16.
Acknowledgment. We are grateful to Dr. Yokoyama, Mitsubishi Heavy Industry, for his ESCA measurement. Registry No. CoTPP, 14172-90-8; TiOz, 13463-67-7; NO, 10102-43-9; Hz,1333-74-0; CO, 630-08-0; N20, 10024-97-2.
Preferentlal Physisorptlon of Orthohydrogen over Parahydrogen by Sodium Mordenite at 77 and 90 K. The Role of Molecular Quadrupole-Field Gradient Interaction for the Hindered Rotation of Sorbed Hydrogen Shoro Furuyama' and Hitoshl Inoue Department of Chemlstw Facuw of Science, Okayama Univers/!y,Okayama 700, Japan (Received: July 9, 1982; I n Final Form: November 23, 1982)
The sorption of H2by iron-free (
